Expression of Notch signaling pathway genes in mouse embryos lacking beta4galactosyltransferase-1

In vivo evidence for GDP-fucose transport in the absence of transporter SLC35C1 and putative transporter SLC35C2

Slc35c1 encodes an antiporter that transports GDP-fucose into the Golgi and returns GMP to the cytoplasm. The closely related gene Slc35c2 encodes a putative GDP-fucose transporter and promotes Notch fucosylation and Notch signaling in cultured cells. Here, we show that HEK293T cells lacking SLC35C1 transferred reduced amounts of O-fucose to secreted epidermal growth factor-like repeats from NOTCH1 or secreted thrombospondin type I repeats from thrombospondin 1. However, cells lacking SLC35C2 did not exhibit reduced fucosylation of these epidermal growth factor-like repeats or thrombospondin type I repeats. To investigate SLC35C2 functions in vivo, WW6 embryonic stem cells were targeted for Slc35c2. Slc35c2[-/-] mice were viable and fertile and exhibited no evidence of defective Notch signaling during skeletal or T cell development. By contrast, mice with inactivated Slc35c1 exhibited perinatal lethality and marked skeletal defects in late embryogenesis, typical of defective Notch signaling. Compound Slc35c1[-/-]Slc35c2[-/-] mutants were indistinguishable in skeletal phenotype from Slc35c1[-/-] embryos and neonates. Double mutants did not exhibit the exacerbated skeletal defects predicted if SLC35C2 was functionally important for Notch signaling in vivo. In addition, NOTCH1 immunoprecipitated from Slc35c1[-/-]Slc35c2[-/-] neonatal lung carried fucose detected by binding of Aleuria aurantia lectin. Given that the absence of both SLC35C1, a known GDP-fucose transporter, and SLC35C2, a putative GDP-fucose transporter, did not lead to afucosylated NOTCH1 nor to the severe Notch signaling defects and embryonic lethality expected if all GDP-fucose transport were abrogated, at least one more mechanism of GDP-fucose transport into the secretory pathway must exist in mammals.

Decreased B4GALT1 promotes hepatocellular carcinoma cell invasiveness by regulating the laminin-integrin pathway

Beta1,4-galactosyltransferases (B4GALTs) play a crucial role in several diseases, including cancer. B4GALT1 is highly expressed in the liver, and patients with mutations in B4GALT1 exhibit hepatopathy. However, the role of B4GALT1 in liver cancer remains unclear. Here, we found that B4GALT1 was significantly downregulated in hepatocellular carcinoma (HCC) tissue compared with the adjacent liver tissue, and low B4GALT1 expression was associated with vascular invasion and poor overall survival in patients with HCC. Additionally, silencing or loss of B4GALT1 enhanced HCC cell migration and invasion in vitro and promoted lung metastasis of HCC in NOD/SCID mice. Moreover, B4GALT1 knockdown or knockout increased cell adhesion to laminin, whereas B4GALT1 overexpression decreased the adhesion. Through a mass spectrometry-based approach and Griffonia simplicifolia lectin II (GSL-II) pull-down assays, we identified integrins α6 and β1 as the main protein substrates of B4GALT1 and their N-glycans were modified by B4GALT1. Further, the increased cell migration and invasion induced by B4GALT1 knockdown or knockout were significantly reversed using a blocking antibody against integrin α6 or integrin β1. These results suggest that B4GALT1 downregulation alters N-glycosylation and enhances the laminin-binding activity of integrin α6 and integrin β1 to promote invasiveness of HCC cells. Our findings provide novel insights into the role of B4GALT1 in HCC metastasis and highlight targeting the laminin-integrin axis as a potential therapeutic strategy for HCC with low B4GALT1 expression.

Galactosyltransferase B4GALT1 confers chemoresistance in pancreatic ductal adenocarcinomas by upregulating N-linked glycosylation of CDK11p110

Aberrant glycosylation in pancreatic cancer has been linked to cancer development, progression and chemoresistance. However, the role of glycogene, such as galactosyltransferase, in pancreatic cancer remains unknown. Herein, we establish beta-1.4-galactosyltransferase 1 (B4GALT1) as a clinical marker and regulator of chemoresistance. Clinically, high B4GALT1 expression correlates with poor survival, enhanced tumor size, increased lymph node metastasis, elevated cancer progression and enhanced incidence of relapse in PDAC patients. Expression of B4GALT1 is up-regulated in gemcitabine resistant patient derived organoids as well as chemoresistant cancer cell lines, while genetic perturbation of its expression in PDAC cell lines regulates cancer progression and chemoresistance. Mechanistically, we show that elevated p65 activity transcriptionally up-regulates B4GALT1 expression, which then interacts with and stabilizes cyclin dependent kinase 11 isomer CDK11^p110 protein via N-linked glycosylation, in order to promote cancer progression and chemoresistance. Finally, depletion of B4GALT1 rescues the response of chemoresistant cells to gemcitabine in an orthotopic PDAC model. Overall, our data uncovers a mechanism by which p65-B4GALT1-CDK11^p110 signalling axis determines cancer progression and chemoresistance, providing a new therapeutic target for an improved pancreatic cancer treatment.

Xylosyl Extension of O-Glucose Glycans on the Extracellular Domain of NOTCH1 and NOTCH2 Regulates Notch Cell Surface Trafficking

Biochemical and genetic studies have indicated that O-linked glycosylation such as O-glucose (Glc), fucose (Fuc), and N-acetylglucosamine (GlcNAc) is critical for Notch signaling; however, it is not fully understood how O-glycans regulate the Notch receptor function. Notch receptors are type-I transmembrane proteins with large extracellular domains (ECD), containing 29–36 epidermal growth factor-like (EGF) repeats. Here, we analyzed O-Glc glycans on NOTCH1 and NOTCH2 expressed in HEK293T cells using an Orbitrap Fusion mass spectrometer and successfully revealed the structures and stoichiometries of all 17 EGF repeats of NOTCH1 with the O-Glc consensus sequence (C1-X-S-X-(P/A)-C2), and 16 out of 17 EGF repeats of NOTCH2 with the same consensus sequence. High levels of O-Glc attachment and xylosyl elongation were detected on most NOTCH1 and NOTCH2 EGF repeats. When both glucoside xylosyltransferases, GXYLT1 and GXYLT2, responsible for the xylosyl elongation of O-glucose, were genetically deleted, the expression of endogenous NOTCH1 and NOTCH2 on the surface of HEK293T cells did not change, but the cell surface expression of overexpressed NOTCH1 and NOTCH2 decreased compared with that in the wild type cells. In vitro secretion assays consistently showed a reduced secretion of both the NOTCH1 and NOTCH2 ECDs in GXYLT1 and GXYLT2 double knockout cells compared with the wild type cells, suggesting a significant role of the elongation of O-Glc glycans on the Notch ECDs in the quality control of Notch receptors.

Effects of Notch glycosylation on health and diseases

Notch signaling is an evolutionarily conserved signaling pathway and is essential for cell-fate specification in metazoans. Dysregulation of Notch signaling results in various human diseases, including cardiovascular defects and cancer. In 2000, Fringe, a known regulator of Notch signaling, was discovered as a Notch-modifying glycosyltransferase. Since then, glycosylation-a post-translational modification involving literal sugars-on the Notch extracellular domain has been noted as a critical mechanism for the regulation of Notch signaling. Additionally, the presence of diverse O-glycans decorating Notch receptors has been revealed in the extracellular domain epidermal growth factor-like (EGF) repeats. Here, we concisely summarize the recent studies in the human diseases associated with aberrant Notch glycosylation.

A modifier in the 129S2/SvPasCrl genome is responsible for the viability of Notch1[12f/12f] mice

Background

Mouse NOTCH1 carries a highly conserved O-fucose glycan at Thr466 in epidermal growth factor-like repeat 12 (EGF12) of the extracellular domain. O-Fucose at this site has been shown by X-ray crystallography to be recognized by both DLL4 and JAG1 Notch ligands. We previously showed that a Notch1 Thr466Ala mutant exhibits very little ligand-induced NOTCH1 signaling in a reporter assay, whereas a Thr466Ser mutation enables the transfer of O-fucose and reverts the NOTCH1 signaling defect. We subsequently generated a mutant mouse with the Thr466Ala mutation termed Notch1[12f](Notch1^tm2Pst). Surprisingly, homozygous Notch1[12f/12f] mutants on a mixed background were viable and fertile.

Results

We now report that after backcrossing to C57BL/6 J mice for 11–15 generations, few homozygous Notch1[12f/12f] embryos were born. Timed mating showed that embryonic lethality occurred by embryonic day (E) ~E11.5, somewhat delayed compared to mice lacking Notch1 or Pofut1 (the O-fucosyltransferase that adds O-fucose to Notch receptors), which die at ~E9.5. The phenotype of C57BL/6 J Notch1[12f/12f] embryos was milder than mutants affected by loss of a canonical Notch pathway member, but disorganized vasculogenesis in the yolk sac, delayed somitogenesis and development were characteristic. In situ hybridization of Notch target genes Uncx4.1 and Dll3 or western blot analysis of NOTCH1 cleavage did not reveal significant differences at E9.5. However, qRT-PCR of head cDNA showed increased expression of Dll3, Uncx4.1 and Notch1 in E9.5 Notch1[12f/12f] embryos. Sequencing of cDNA from Notch1[12f/12f] embryo heads and Southern analysis showed that the Notch1[12f] locus was intact following backcrossing. We therefore looked for evidence of modifying gene(s) by crossing C57BL/6 J Notch1 [12f/+] mice to 129S2/SvPasCrl mice. Intercrosses of the F1 progeny gave viable F2 Notch1[12f/12f] mice.

Conclusion

We conclude that the 129S2/SvPasCrl genome contains a dominant modifying gene that rescues the functions of NOTCH1[12f] in signaling. Identification of the modifying gene has the potential to illuminate novel factor(s) that promote Notch signaling when an O-fucose glycan is absent from EGF12 of NOTCH1.

SLC35A2-CDG: Functional characterization, expanded molecular, clinical, and biochemical phenotypes of 30 unreported Individuals

Pathogenic de novo variants in the X-linked gene SLC35A2 encoding the major Golgi-localized UDP-galactose transporter required for proper protein and lipid glycosylation cause a rare type of congenital disorder of glycosylation known as SLC35A2-congenital disorders of glycosylation (CDG; formerly CDG-IIm). To date, 29 unique de novo variants from 32 unrelated individuals have been described in the literature. The majority of affected individuals are primarily characterized by varying degrees of neurological impairments with or without skeletal abnormalities. Surprisingly, most affected individuals do not show abnormalities in serum transferrin N-glycosylation, a common biomarker for most types of CDG. Here we present data characterizing 30 individuals and add 26 new variants, the single largest study involving SLC35A2-CDG. The great majority of these individuals had normal transferrin glycosylation. In addition, expanding the molecular and clinical spectrum of this rare disorder, we developed a robust and reliable biochemical assay to assess SLC35A2-dependent UDP-galactose transport activity in primary fibroblasts. Finally, we show that transport activity is directly correlated to the ratio of wild-type to mutant alleles in fibroblasts from affected individuals.

Multiple roles for O-glycans in Notch signalling

Notch signalling regulates a plethora of developmental processes and is also essential for the maintenance of tissue homeostasis in adults. Therefore, fine-tuning of Notch signalling strength needs to be tightly regulated. Of key importance for the regulation of Notch signalling are O-fucose, O-GlcNAc and O-glucose glycans attached to the extracellular domain of Notch receptors. The EGF repeats of the Notch receptor extracellular domain harbour consensus sites for addition of the different types of O-glycan to Ser or Thr, which takes place in the endoplasmic reticulum. Studies from Drosophila to mammals have demonstrated the multifaceted roles of O-glycosylation in regulating Notch signalling. O-glycosylation modulates different aspects of Notch signalling including recognition by Notch ligands, the strength of ligand binding, Notch receptor trafficking, stability and activation at the cell surface. Defects in O-glycosylation of Notch receptors give rise to pathologies in humans. This Review summarizes the nature of the O-glycans on Notch receptors and their differential effects on Notch signalling.

Structural Divergence in O-GlcNAc Glycans Displayed on Epidermal Growth Factor-like Repeats of Mammalian Notch1

Extracellular O-GlcNAc is a novel class of modification catalyzed by epidermal growth factor-like (EGF)-domain specific O-GlcNAc transferase (EOGT). In mammals, EOGT is required for ligand-mediated Notch signaling for vascular development. Previous studies have revealed that O-GlcNAc in mammalian cultured cells is subject to subsequent glycosylation, which may impose additional layers of regulation. This study aimed to analyze the O-GlcNAc glycans of Drosophila EGF20 as model substrates and mouse Notch1 EGF repeats by mass-spectrometry. The analysis of Drosophila EGF20 expressed in HEK293T cells revealed that the majority of the proteins are modified with an elongated form of O-GlcNAc glycan comprising terminal galactose or sialic acid residues. In contrast, recombinant Notch1 EGF repeats isolated from HEK293T cells revealed structural divergence of O-GlcNAc glycans among the different EGF domains. Although the majority of Notch1 EGF2 and EGF20 domains contained the extended forms of the glycan, the O-GlcNAc in many other domains mostly existed as a monosaccharide irrespective of the exogenous EOGT expression. Our results raised a hypothesis that an array of O-GlcNAc monosaccharides may impact the structure and function of Notch receptors.

Regulation of Notch Function by O-Glycosylation

The Notch receptor initiates a unique intercellular signaling pathway that is evolutionarily conserved across all metazoans and contributes to the development and maintenance of numerous tissues. Consequently, many diseases result from aberrant Notch signaling. Emerging roles for Notch in disease are being uncovered as studies reveal new information regarding various components of this signaling pathway. Notch activity is regulated at several levels, but O-linked glycosylation of Epidermal Growth Factor (EGF) repeats in the Notch extracellular domain has emerged as a major regulator that, depending on context, can increase or decrease Notch activity. Three types of O-linked glycosylation occur at consensus sequences found within the EGF repeats of Notch: O-fucosylation, O-glucosylation, and O-GlcNAcylation. Recent studies have investigated the site occupancy of these types of glycosylation and also defined specific roles for these glycans on Notch structure and function. Nevertheless, there are many functional aspects to each type of O-glycosylation that remain unclear. Here, we will discuss molecular mechanisms of how O-glycosylation regulates Notch signaling and describe disorders associated with defects in Notch O-glycosylation.

Glioma cell fate decisions mediated by Dll1-Jag1-Fringe in Notch1 signaling pathway

BACKGROUND

The Notch family of proteins plays a vital role in determining cell fates, such as proliferation, differentiation, and apoptosis. It has been shown that Notch1 and its ligands, Dll1 and Jag1, are overexpressed in many glioma cell lines and primary human gliomas. The roles of Notch1 in some cancers have been firmly established, and recent data implicate that it plays important roles in glioma cell fate decisions. This paper focuses on devising a specific theoretical framework that incorporates Dll1, Jag1, and Fringe in Notch1 signaling pathway to explore their functional roles of these proteins in glioma cells in the tumorigenesis and progression of human gliomas, and to study how glioma cell fate decisions are modulated by both trans-activation and cis-inhibition.

RESULTS

This paper presents a computational model for Notch1 signaling pathway in glioma cells. Based on the bifurcation analysis of the model, we show that how the glioma cell fate decisions are modulated by both trans-activation and cis-inhibition mediated by the Fringe protein, providing insight into the design and control principles of the Notch signaling system and the gliomas.

CONCLUSIONS

This paper presents a computational model for Notch1 signaling pathway in glioma cells based on intertwined dynamics with cis-inhibition and trans-activation involving the proteins Notch1, Dll1, Jag1, and Fringe. The results show that how the glioma cell fate transitions are performed by the Notch1 signaling. Transition from grade III ∼ IV with significantly high Notch1 to grade I ∼ II with high Notch1, and then to normal cells by repressing the Fringe levels or decreasing the strength of enhancement induced by Fringe.

O-GlcNAc on NOTCH1 EGF repeats regulates ligand-induced Notch signaling and vascular development in mammals

The glycosyltransferase EOGT transfers O-GlcNAc to a consensus site in epidermal growth factor-like (EGF) repeats of a limited number of secreted and membrane proteins, including Notch receptors. In EOGT-deficient cells, the binding of DLL1 and DLL4, but not JAG1, canonical Notch ligands was reduced, and ligand-induced Notch signaling was impaired. Mutagenesis of O-GlcNAc sites on NOTCH1 also resulted in decreased binding of DLL4. EOGT functions were investigated in retinal angiogenesis that depends on Notch signaling. Global or endothelial cell-specific deletion of Eogt resulted in defective retinal angiogenesis, with a mild phenotype similar to that caused by reduced Notch signaling in retina. Combined deficiency of different Notch1 mutant alleles exacerbated the abnormalities in Eogt-/- retina, and Notch target gene expression was decreased in Eogt-/-endothelial cells. Thus, O-GlcNAc on EGF repeats of Notch receptors mediates ligand-induced Notch signaling required in endothelial cells for optimal vascular development.

What Have We Learned from Glycosyltransferase Knockouts in Mice?

There are five major classes of glycan including N- and O-glycans, glycosaminoglycans, glycosphingolipids, and glycophosphatidylinositol anchors, all expressed at the molecular frontier of each mammalian cell. Numerous biological consequences of altering the expression of mammalian glycans are understood at a mechanistic level, but many more remain to be characterized. Mouse mutants with deleted, defective, or misexpressed genes that encode activities necessary for glycosylation have led the way to identifying key functions of glycans in biology. However, with the advent of exome sequencing, humans with mutations in genes involved in glycosylation are also revealing specific requirements for glycans in mammalian development. The aim of this review is to summarize glycosylation genes that are necessary for mouse embryonic development, pathway-specific glycosylation genes whose deletion leads to postnatal morbidity, and glycosylation genes for which effects are mild, but perturbation of the organism may reveal functional consequences. General strategies for generating and interpreting the phenotype of mice with glycosylation defects are discussed in relation to human congenital disorders of glycosylation (CDG).

The multiple roles of epidermal growth factor repeat O-glycans in animal development

The epidermal growth factor (EGF)-like repeat is a common, evolutionarily conserved motif found in secreted proteins and the extracellular domain of transmembrane proteins. EGF repeats harbor six cysteine residues which form three disulfide bonds and help generate the three-dimensional structure of the EGF repeat. A subset of EGF repeats harbor consensus sequences for the addition of one or more specific O-glycans, which are initiated by O-glucose, O-fucose or O-N-acetylglucosamine. These glycans are relatively rare compared to mucin-type O-glycans. However, genetic experiments in model organisms and cell-based assays indicate that at least some of the glycosyltransferases involved in the addition of O-glycans to EGF repeats play important roles in animal development. These studies, combined with state-of-the-art biochemical and structural biology experiments have started to provide an in-depth picture of how these glycans regulate the function of the proteins to which they are linked. In this review, we will discuss the biological roles assigned to EGF repeat O-glycans and the corresponding glycosyltransferases. Since Notch receptors are the best studied proteins with biologically-relevant O-glycans on EGF repeats, a significant part of this review is devoted to the role of these glycans in the regulation of the Notch signaling pathway. We also discuss recently identified proteins other than Notch which depend on EGF repeat glycans to function properly. Several glycosyltransferases involved in the addition or elongation of O-glycans on EGF repeats are mutated in human diseases. Therefore, mechanistic understanding of the functional roles of these carbohydrate modifications is of interest from both basic science and translational perspectives.

Novel roles for O-linked glycans in protein folding

N-Glycosylation has long been linked to protein folding and quality control in the endoplasmic reticulum (ER). Recent work has shown that O-linked glycosylation and the corresponding glycosyltransferases also participate in this important function. Notably, Protein O-fucosyltransferase 1 (Ofut1/Pofut1), a soluble, ER localized enzyme that fucosylates Epidermal Growth Factor-like (EGF) repeats, functions as a chaperone involved in the proper localization of the Notch receptor in certain contexts. Pofut2, a related enzyme that modifies Thrombospondin type I repeats (TSRs), has also been hypothesized to play a role in the folding and quality control of TSR-containing proteins. Both enzymes only modify fully folded substrates suggesting that they are able to distinguish between folded and unfolded structures. Pofuts have known physiological relevance and are conserved across metazoans. Though consensus sequences for O-fucosylation have been established and structures of both Pofuts have been studied, the mechanism of how they participate in protein folding is not known. This article discusses past and recent advances made in novel roles for these protein O-glycosyltransferases.

B4GALT1 gene knockdown inhibits the hedgehog pathway and reverses multidrug resistance in the human leukemia K562/adriamycin-resistant cell line

B4GALT1 gene encodes type II membrane-bound glycoprotein, named β-1, 4-galactosyltransferase 1 (β1, 4-Gal-T1), which can transfer galactose to acceptor sugars. B4GALT1 gene plays important roles in physiological process and disease development. In this study, we investigate the possible role and mechanism of B4GALT1 gene in multidrug resistance of human leukemia cell line. Significantly, higher expression of B4GALT1 was observed in adriamycin-resistant (ADR) K562 cell line (K562/ADR) than that in K562 cell line by real-time polymerase chain reaction and Western blotting. The activity of β1, 4-Gal-T1 enzyme, and Galβ-1,4GlcNAc structures on cell membrane glycoproteins was found at higher levels in K562/ADR cells than those in K562 cells. Further analysis of the B4GALT1 deregulation after using RNA interference approach showed that the silencing of B4GALT1 in K562/ADR cells resulted in increased sensitivity to chemotherapeutic drugs both in vitro and in vivo. The activity of the hedgehog signaling pathway affected the chemosensitivity of K562/ADR cells and was downregulated in K562/ADR cells with suppression of B4GALT1 gene. We hypothesize that B4GALT1 is responsible for the overcoming multidrug resistance in human leukemia therapy via regulating the activity of the hedgehog signaling pathway.

B4GALT1-congenital disorders of glycosylation presents as a non-neurologic glycosylation disorder with hepatointestinal involvement

The clinical phenotype of congenital disorders of glycosylation is heterogeneous, mostly including a severe neurological involvement and multisystem disease. We identified a novel patient with a galactosyltransferase deficiency with mild hepatopathy and coagulation anomalies, but normal psychomotor development. The tissue-specific expression of the defective B4GALT1 gene correlated with the clinical phenotype.

Galactose differentially modulates lunatic and manic fringe effects on Delta1-induced NOTCH signaling

NOTCH signaling induced by Delta1 (DLL1) and Jagged1 (JAG1) NOTCH ligands is modulated by the β3N-acetylglucosaminyl transferase Fringe. LFNG (Lunatic Fringe) and MFNG (Manic Fringe) transfer N-acetylglucosamine (GlcNAc) to O-fucose attached to EGF-like repeats of NOTCH receptors. In co-culture NOTCH signaling assays, LFNG generally enhances DLL1-induced, but inhibits JAG1-induced, NOTCH signaling. In mutant Chinese hamster ovary (CHO) cells that do not add galactose (Gal) to the GlcNAc transferred by Fringe, JAG1-induced NOTCH signaling is not inhibited by LFNG or MFNG. In mouse embryos lacking B4galt1, NOTCH signaling is subtly reduced during somitogenesis. Here we show that DLL1-induced NOTCH signaling in CHO cells was enhanced by LFNG, but this did not occur in either Lec8 or Lec20 CHO mutants lacking Gal on O-fucose glycans. Lec20 mutants corrected with a B4galt1 cDNA became responsive to LFNG. By contrast, MFNG promoted DLL1-induced NOTCH signaling better in the absence of Gal than in its presence. This effect was reversed in Lec8 cells corrected by expression of a UDP-Gal transporter cDNA. The MFNG effect was abolished by a DDD to DDA mutation that inactivates MFNG GlcNAc transferase activity. The binding of soluble NOTCH ligands and NOTCH1/EGF1-36 generally reflected changes in NOTCH signaling caused by LFNG and MFNG. Therefore, the presence of Gal on O-fucose glycans differentially affects DLL1-induced NOTCH signaling modulated by LFNG versus MFNG. Gal enhances the effect of LFNG but inhibits the effect of MFNG on DLL1-induced NOTCH signaling, with functional consequences for regulating the strength of NOTCH signaling.

Fringe benefits: functional and structural impacts of O-glycosylation on the extracellular domain of Notch receptors

The Notch family of receptors plays essential roles in many phases of development, and dysregulation of Notch activity is increasingly recognized as a player in many diseases. O-Glycosylation of the Notch extracellular domain is essential for Notch activity, and tissue-specific alterations in the glycan structures are known to regulate activity. Here we review recent advances in identification and characterization of the enzymes responsible for glycosylating Notch and molecular mechanisms for how these O-glycans affect Notch activity.

Role of glycans and glycosyltransferases in the regulation of Notch signaling

The evolutionarily conserved Notch signaling pathway plays broad and important roles during embryonic development and in adult tissue homeostasis. Unlike most other pathways used during animal development, Notch signaling does not rely on second messengers and intracellular signaling cascades. Instead, pathway activation results in the cleavage of the Notch intracellular domain and its translocation into the nucleus, where it functions as a transcriptional co-activator of the Notch target genes. To ensure tight spatial and temporal regulation of a pathway with such an unusually direct signaling transduction, animal cells have devised a variety of specialized modulatory mechanisms. One such mechanism takes advantage of decorating the Notch extracellular domain with rare types of O-linked glycans. In this review, we will discuss the genetic and biochemical data supporting the notion that carbohydrate modification is essential for Notch signaling and attempt to provide a brief historical overview of how we have learned what we know about the glycobiology of Notch. We will also summarize what is known about the contribution of specific nucleotide-sugar transporters to Notch biology and the roles-enzymatic and non-enzymatic-played by specific glycosyltransferases in the regulation of this pathway. Mutations in the Notch pathway components cause a variety of human diseases, and manipulation of Notch signaling is emerging as a powerful tool in regenerative medicine. Therefore, studying how sugar modification modulates Notch signaling provides a framework for better understanding the role of glycosylation in animal development and might offer new tools to manipulate Notch signaling for therapeutic purposes.

Role of glycosylation of Notch in development

The Notch pathway is one of the major signaling pathways required for proper development in metazoans. Notch activity is regulated at numerous levels, and increasing evidence reveals the importance of "protein glycosylation" (modification of Notch receptors with sugars) for its regulation. In this review we summarize the significance of the Notch pathway in development and the players responsible for its glycosylation, and then discuss the molecular mechanisms by which protein glycosylation may regulate Notch function.

Roles of glycosylation in Notch signaling

Notch and the DSL Notch ligands Delta and Serrate/Jagged are glycoproteins with a single transmembrane domain. The extracellular domain (ECD) of both Notch receptors and Notch ligands contains numerous epidermal growth factor (EGF)-like repeats which are post-translationally modified by a variety of glycans. Inactivation of a subset of genes that encode glycosyltransferases which initiate and elongate these glycans inhibits Notch signaling. In the formation of developmental boundaries in Drosophila and mammals, in mouse T-cell and marginal zone B-cell development, and in co-culture Notch signaling assays, the regulation of Notch signaling by glycans is to date a cell-autonomous effect of the Notch-expressing cell. The regulation of Notch signaling by glycans represents a new paradigm of signal transduction. O-fucose glycans modulate the strength of Notch binding to DSL Notch ligands, while O-glucose glycans facilitate juxta-membrane cleavage of Notch, generating the substrate for intramembrane cleavage and Notch activation. Identifying precisely how the addition of particular sugars at specific locations on Notch modifies Notch signaling is a challenge for the future.

Regulation of Notch signaling during T- and B-cell development by O-fucose glycans

Notch signaling is required for the development of all T cells and marginal zone (MZ) B cells. Specific roles in T- and B-cell differentiation have been identified for different Notch receptors, the canonical Delta-like (Dll) and Jagged (Jag) Notch ligands, and downstream effectors of Notch signaling. Notch receptors and ligands are post-translationally modified by the addition of glycans to extracellular domain epidermal growth factor-like (EGF) repeats. The O-fucose glycans of Notch cell-autonomously modulate Notch-ligand interactions and the strength of Notch signaling. These glycans are initiated by protein O-fucosyltransferase 1 (Pofut1), and elongated by the transfer of N-acetylglucosamine (GlcNAc) to the fucose by beta1,3GlcNAc-transferases termed lunatic, manic, or radical fringe. This review discusses T- and B-cell development from progenitors deficient in O-fucose glycans. The combined data show that Lfng and Mfng regulate T-cell development by enhancing the interactions of Notch1 in T-cell progenitors with Dll4 on thymic epithelial cells. In the spleen, Lfng and Mfng cooperate to modify Notch2 in MZ B progenitors, enhancing their interaction with Dll1 on endothelial cells and regulating MZ B-cell production. Removal of O-fucose affects Notch signaling in myelopoiesis and lymphopoiesis, and the O-fucose glycan in the Notch1 ligand-binding domain is required for optimal T-cell development.

Role of unusual O-glycans in intercellular signaling

In the last two decades, our knowledge of the role of glycans in development and signal transduction has expanded enormously. While most work has focused on the importance of N-linked or mucin-type O-linked glycosylation, recent work has highlighted the importance of several more unusual forms of glycosylation that are the focus of this review. In particular, the ability of O-fucose glycans on the epidermal growth factor-like (EGF) repeats of Notch to modulate signaling places glycosylation alongside phosphorylation as a means to modulate protein-protein interactions and their resultant downstream signals. The recent discovery that O-glucose modification of Notch EGF repeats is also required for Notch function has further expanded the range of glycosylation events capable of modulating Notch signaling. The prominent role of Notch during development and in later cell-fate decisions underscores the importance of these modifications in human biology. The role of glycans in intercellular signaling events is only beginning to be understood and appears ready to expand into new areas with the discovery that thrombospondin type 1 repeats are also modified with O-fucose glycans. Finally, a rare form of glycosylation called C-mannosylation modifies tryptophans in some signaling competent molecules and may be a further layer of complexity in the field. We will review each of these areas focusing on the glycan structures produced, the consequence of their presence, and the enzymes responsible.

Regulation of Notch signaling by glycosylation

Notch receptors are approximately 300 kDa cell surface glycoproteins whose activation by Notch ligands regulates cell fate decisions in the metazoa. The extracellular domain of Notch receptors has many epidermal growth factor like repeats that are glycosylated with O-fucose and O-glucose glycans as well as N-glycans. Disruption of O-fucose glycan synthesis leads to severe Notch signaling defects in Drosophila and mammals. Removal or addition of O-fucose glycan consensus sites on Notch receptors also leads to Notch signaling defects. Ligand binding and ligand-induced Notch signaling assays have provided insights into how changes in the O-fucose glycans of Notch receptors alter Notch signaling.

Functional roles for beta1,4-N-acetlygalactosaminyltransferase-A in Drosophila larval neurons and muscles

Adult Drosophila mutant for the glycosyltransferase beta1,4-N-acetlygalactosaminyltransferase-A (beta4GalNAcTA) display an abnormal locomotion phenotype, indicating a role for this enzyme, and the glycan structures that it generates, in the neuromuscular system. To investigate the functional role of this enzyme in more detail, we turned to the accessible larval neuromuscular system and report here that larvae mutant for beta4GalNAcTA display distinct nerve and muscle phenotypes. Mutant larvae exhibit abnormal backward crawling, reductions in nerve terminal bouton number, decreased spontaneous transmitter-release frequency, and short, wide muscles. This muscle shape change appears to result from hypercontraction since the individual sarcomeres are shorter in mutant muscles. Analysis of muscle calcium signals showed altered calcium handling in the mutant, suggesting a mechanism by which hypercontraction could occur. All of these phenotypes can be rescued by a transgene carrying the beta4GalNAcTA genomic region. Tissue-specific expression, using the Gal4-UAS system, reveals that neural expression rescues the mutant crawling phenotype, while muscle expression rescues the muscle defect. Tissue-specific expression did not appear to rescue the decrease in neuromuscular junction bouton number, suggesting that this defect arises from cooperation between nerve and muscle. Altogether, these results suggest that beta4GalNAcTA has at least three distinct functional roles.

A beta1,4-galactosyltransferase is required for convergent extension movements in zebrafish

Our understanding of how complex carbohydrates function during embryonic development is still very limited, primarily due to the large number of glycosyltransferases now known to be involved in their synthesis. To overcome these limitations, we have taken advantage of the zebrafish system to analyze the function of complex carbohydrates during development by down-regulating the expression of specific glycosyltransferases. Herein, we report the identification of the zebrafish ortholog of mammalian beta1,4-galactosyltransferase I, beta4GalT1, and its requirement for proper convergent extension movements during gastrulation. beta4GalT1 is expressed in the oocyte and throughout the embryo during the first 24 h of development. Knockdown of zebrafish beta4GalT1 by two independent morpholino oligonucleotides results in embryos with a truncated anterior-posterior axis, as well as elongated somites and moderate defects in the patterning of the head mesenchyme. Co-injection of zebrafish beta4GalT1 mRNA returns galactosyltransferase activity to control levels and rescues the defects produced by morpholino oligonucleotides. In situ hybridizations of various molecular markers reveal that the axial mesoderm of epiboly stage embryos is abnormally widened in beta4GalT1 morphants, indicative of abnormal convergent extension. Consistent with this, the rate of anterior-posterior axis elongation is reduced relative to control-injected embryos, similar to that seen in known convergent extension mutants. Among the many potential substrates for beta4GalT1 is laminin, a principle component of the extracellular matrix that supports cell movements such as those that occur during convergent extension. Previous in vitro studies have shown that the galactosylation status of laminin directly influences its ability to support cell spreading and migration. In this regard, laminin isolated from beta4GalT1 morphant embryos is poorly galactosylated, which may contribute to defective cell migration during convergent extension movements. This work demonstrates that zebrafish can be used to identify critical developmental roles for specific glycosyltransferases that would not be obvious otherwise, such as an absolute requirement for beta4GalT1 during convergent extension movements.

Roles of O-fucose glycans in notch signaling revealed by mutant mice

Notch receptor signaling is important for many developmental processes in the metazoa. Insights into how Notch receptor signaling is regulated have been obtained from the characterization of mutants of model organisms in which Notch signaling is perturbed. Here we describe the effects of mutations that alter the glycosylation of Notch receptors and Notch ligands in the mouse. The extracellular domain of Notch receptors and Notch ligands carries N-glycans and O-glycans, including O-fucose and O-glucose glycans. Mutations in several genes that inhibit the synthesis of O-fucose glycans, and one that also affects the maturation of N-glycans, cause Notch signaling defects and disrupt development.